As a reversible electromagnetic contactor that is connected to a power feeding circuit of an induction motor and performs a normal and reverse operation control of the induction motor, there is known a device in which a reversible unit is mounted so as to extend over two electromagnetic contactors disposed adjacent to each other and mechanically locks the operation in which two electromagnetic contactors simultaneously enter a closed-circuit (ON) state (for example, Patent Document 1).
FIGS. 11 to 14 specifically illustrate a conventional reversible electromagnetic contactor.
As shown in FIG. 11, two electromagnetic contactors 1a and 1b disposed adjacent to each other have a structure in which display windows 3a and 3b are respectively provided in arc-extinguishing covers 2a and 2b and operation display pieces 4a and 4b are respectively provided inside the display windows 3a and 3b to protrude from the inside of the device.
As shown in FIG. 12, the operation display piece 4a of one electromagnetic contactor 1a is a member which is fixed to a movable contact support 4 disposed inside the electromagnetic contactor 1a and extends toward the inside of the display window 3a. Then, the operation display piece may check an operation of the movable contact support 4 since the position of the operation display piece 4a changes inside the display window 3a as well as a closed-circuit (ON) operation and an opened-circuit (OFF) operation of the movable contact support 4. Further, since the position of the operation display piece 4b of the other electromagnetic contactor 1b also changes inside the display window 3b, a closed-circuit (ON) operation and an opened-circuit (OFF) operation of a movable contact support (not shown) disposed inside the electromagnetic contactor 1b may be checked.
As shown in FIG. 11, a reversible unit 6 includes a unit bottom plate 6a which is connected to the arc-extinguishing covers 2a and 2b in such a state as to extend over two electromagnetic contactors 1a and 1b and a unit cover 6b, a peripheral edge portion of which engages a peripheral edge of the unit bottom plate 6a, where in the internal space of the unit bottom plate 6a and the unit cover 6b, first and second rotation plates 6c and 6d are rotatably connected to a rotation regulation member 6e. Further, cylindrical connection pieces 6f and 6g are respectively formed at the end portions of the first rotation plate 6c and the second rotation plate 6d to protrude outward through open portions 6h and 6i formed in the bottom plate 6a. Then, a plurality of hook portions 6j is formed at the unit bottom plate 6a, and a plurality of engagement holes 2c is formed in the arc-extinguishing covers 2a and 2b to correspond to the hook portions 6j. When the unit bottom plate 6a contacts the arc-extinguishing covers 2a and 2b, the plurality of hook portions 6j detachably engages with the corresponding engagement holes 2c. 
Then, when the connection pieces 6f and 6g of the reversible unit 6 are fitted to the head portions of the operation display pieces 4a and 4b of two electromagnetic contactors 1a and 1b as shown in FIG. 13, the rotation regulation member 6e rotates one of the first rotation plate 6c and the second rotation plate 6d, restricts the rotation of the other thereof, permits the movement of only one of the operation display pieces 4a and 4b, and thus mechanically locks the operation in which two electromagnetic contactors 1a and 1b simultaneously enter a closed-circuit (ON) state.